Various devices have been developed over the years for imprinting characters and symbols upon paper or other sheet materials. Some of these devices, such as page typesetters, print more than one line at a time while some types of line printers print an entire line at a time. However, most common types of typewriters and printers print one character at a time and move along horizontal tracks across the page. Some of these printers do not require an impact surface since they print by means of thermal activation of chemicals within the paper. The direct impact type of printers, however, require some sort of impact surface against which the paper is held while the print character is impacted against a ribbon interposed between the character and the surface of the paper.
The traditional form of impact surface for character printers has been a rotating cylindrical platen. This platen, typically formed of rubber or other resilient material, serves the dual purposes of gripping the paper to feed it through the machine and of forming an impact surface for character striking. The great majority of all typewriters and printers produced since the late nineteenth century have used these rotating platens.
Modern dot matrix printers have occasionally used a stationary metal impact surface rather than a rotating platen. This is possible with dot matrix printers because the print impact mechanism includes a plurality of metal posts or pins which are driven against the ribbon, paper and the print surface. The metal pins are strong and are not subject to chipping or breakage by the hard surface. Furthermore, the quality of print expected from dot matrix printers is not as high as that expected from full character formation (letter quality) impact printers. Therefore, the rougher resulting appearance of the print, inevitably caused by the hard impact surface, is not detrimental to the performance and output of the printer.
For high quality printing of characters, however, it is desirable to have an impact surface which is at least somewhat resilient, such that the impact causes the paper to mold into the surface slightly and allows the character to be fully formed on the paper. This is especially important with lighter grades of paper. Resilient impact surfaces also significantly improve the lifetime of character printing elements, such as daisy wheels, which are frequently formed of plastic. For these reasons, and others, letter quality printers have continued to use the rotating cylindrical platen.
Platens have disadvantages in that they are relatively expensive to manufacture and difficult to replace in a given machine. They also maintain continuous contact with the paper over a wide surface and are liable to deliver soil or other contamination to the reverse surface of the printing medium during use. Since platens serve the dual purposes of gripping the sheet material for transport through the system and as an impact surface, the hardness of the platen material must be limited to that sufficient to provide proper gripping. This severely limits the lifetime of the platen since the surface will gradually become pocked and degraded by repetitive impacts.
For the purposes of modern data processing printers, the platen function of transporting the paper through the system is no longer necessary. Sophisticated roller wheels and other mechanisms can be utilized. This is particularly true since it is no longer necessary for the operator to continually observe what is being printed on the paper during the printing operation.